BEIJING, Feb 17 (Reuters) - Religious extremists were responsible for a "terrorist assault" on Friday in China's far western region of Xinjiang that left 11 people dead, state media said, adding details about the latest violence to hit the restive area with a large Muslim population.

Xinjiang has been convulsed for years by violence that Beijing blames on Islamist militants and separatists who want to establish an independent state called East Turkestan.

But exiles and many rights groups trace the cause to China's policies, including curbs on Islam and the Uighur people's culture and language, charges the government denies.

An investigation showed the attack in Wushi county, close to China's border with Kyrgyzstan, was an "organized, premeditated terrorist assault targeting the police," state news agency Xinhua said late on Sunday, citing Xinjiang police.

A group of 13 "terrorists" trained for six months under a man named Mehmut Tohti, who had "spread religious extremism" for three years, Xinhua said.

It did not identify him as one of the mainly Muslim Uighur people who call the region home, but his name suggests he is.

Xinhua said the extremists listened to audio products and watched videos of terrorism content before the attack on police vehicles, adding that since January the group had made explosive devices and knives.

The agency earlier reported that the group put LNG cylinders in their car in a bid to use it as a bomb. Eight were killed by police and three died "by their own suicide bomb", it said.

Many vehicles in the region use LNG, or liquefied natural gas, as fuel.

Two civilians and two police officers were injured in the incident and five police vehicles were damaged or destroyed, Xinhua said.

More than 100 people, several policemen among them, have been killed in violence in Xinjiang since last April, state media have reported.

Last month Kyrgyzstan said its border guards had killed 11 people believed to be members of a militant group of Uighurs.

Rights advocates argue that Chinese reports about violence there cannot be trusted and that authorities restrict travel by journalists and do not allow independent and transparent investigations. (Reporting by Michael Martina; Editing by Clarence Fernandez)